1. Field of the Invention
This invention relates to a communication channel establishing method for a communication network which includes a plurality of communication terminals, a plurality of exchanges and a transmission line, and more particularly to a channel establishing method of point-to-multipoint communication.
2. Description of the Related Arts
In a communication network which includes a plurality of exchanges, in order to realize data transfer of a high degree of reliability between calling and called terminals, a connection is established for a communication channel between the calling and called terminals prior to transfer of data. A control procedure to establish a connection is generally called signaling system or signaling, and one of conventional communication systems for realizing point-to-multipoint or multipoint-to-point communication is point-to-multipoint signaling.
A representative one of conventional point-to-multipoint communication systems mentioned above is, for example, a point-to-multipoint connection establishing method for an ATM (Asynchronous Transfer Mode) communication network (refer to ATM Forum UNI (User Network Interface) 3.1).
FIG. 1 is a view illustrating the conventional point-to-multipoint connection establishing method defined in the ATM Forum UNI Signaling 3.1. This establishing method can be applied to a communication network wherein calling terminal 30 and called terminals 31 and 32 are connected to each other via exchanges 300 to 304. In the following description, each communication terminal is referred to as user, and each exchange is referred to as switch.
Referring to FIG. 1, a procedure when calling user 30 establishes a one (calling user) to two (called users) connection to called users 31 and 32 in accordance with the conventional point-to-multipoint connection establishing method defined in the ATM Forum UNI Signaling 3.1.
Calling user 30 sends out a point-to-multipoint connection establishing request consisting of two stages to the communication network. In the first stage request, only first called user 31 is designated. This request is first received by switch 300 in which calling user 30 is accommodated (step C1).
Receiving the request sent out from calling user 30 in step C1, since called user 31 is not accommodated in switch 300, switch 300 selects a route optimum for establishment of a connection to called user 31 and sends out the request to switch 301 in the next stage within the selected route (step C2).
When the request sent out from switch 300 in step C2 is received by switch 301, since called user 31 is not accommodated in switch 301, switch 301 selects a route optimum for establishment of a connection to called user 31 and sends out the request to switch 303 in the next stage within the selected route (step C3).
When the request sent out from switch 301 in step C3 is received by switch 303, since called user 31 is accommodated in switch 303, switch 303 sends out the request to called user 31 (step C4).
Receiving the request sent out from switch 303 in step C4, called user 31 first establishes a connection to switch 303 and then sends out a connection establishment completion message to switch 303 (step C5).
Receiving the connection establishment completion message from called user 31 in step C5, switch 303 establishes a connection between switch 303 and switch 301 and then sends out a connection establishment completion message to switch 301 (step C6).
Receiving the connection establishment completion message from switch 303 in step C6, switch 301 first establishes a connection to switch 300 and then sends out a connection establishment completion message to switch 300 (step C7).
Receiving the connection establishment completion message from switch 301, switch 300 first establishes a connection to calling user 30 and sends out a connection establishment completion message to called user 31 to calling user 30 (step C8).
Receiving the connection establishment completion message to called user 31 from switch 300 in step C8, calling user 30 sends out a second stage request requesting to add another connection to the connection established by the processing in steps C1 to C8 described hereinabove to switch 300 and designates second called user 32 in this second request (step C9).
When the request sent out from calling user 30 in step C9 is received by switch 300, since called user 32 is not accommodated in switch 300, switch 300 selects a route optimum for establishment of a connection to called user 32 and sends out the request to switch 301 in the next stage within the selected route (step C10).
When the request sent out from switch 300 in step C10 is received by switch 301, since called user 32 is not accommodated in switch 301, switch 301 selects a route optimum for establishment of a connection to called user 32 and sends out the request to switch 304 in the next stage which is within the selected route (step C11).
Receiving the request sent out from switch 301 in step C11, since called user 32 is accommodated in switch 304, switch 304 sends out the request to called user 32 (step C12).
Receiving the request sent out from switch 304 in step C12, called user 32 establishes a connection to switch 304 and then sends out a connection establishment completion message to switch 304 (step C13).
Receiving the connection establishment completion message sent out from the called user in step C13, switch 304 establishes a connection between switch 304 and switch 301 and then sends out a connection establishment completion message to switch 301 (step C14).
When the connection establishment completion message sent out from switch 304 in step C14 is received by switch 301, since the connection was established already between switch 300 and switch 301 in step C7, switch 301 does not establish a new connection, but is set so that data sent from switch 300 to switch 301 upon data transmission may be distributed simultaneously to switch 303 and switch 304. Thereafter, switch 301 sends out a connection establishment completion message to switch 300 (step C15).
When the connection establishment completion message sent out from switch 301 in step C15 is received by switch 300, since the connection was established already between switch 300 and calling user 30 in step C8, switch 300 does not establish a new connection, but sends out a connection establishment completion message to called user 32 to calling user 30 (step C16).
After such one (calling user) to two (called users) connection as seen in FIG. 1 is established in accordance with the series of steps C1 to C16 described above, one data sent from calling user 30 upon data transmission is copied by switch 301 which serves as a branching point of the connection and finally reaches two called users 31 and 32.
It is to be noted that a one (calling user) to N (called users) connection can be established with contents of similar steps by repeating steps C9 to C16 described above by N-1 times adding to the steps of C1 to C8.
However, the conventional point-to-multipoint or multipoint-to-point connection establishing method described above has a problem in that, since establishment of a connection is performed sequentially for each one called user, an increasing time is required to complete the establishment in proportion to the number of calling/called users. For example, in the communication network shown in FIG. 1, establishment of a connection to called user 32 cannot be started before a connection to called user 31 is established.